The present invention relates to a transceiver for wirelessly accessing a transmission channel, to a system comprising such a transceiver and to a method for reducing a self-interference of a transceiver. The present invention further relates to an agile full-duplex wireless transceiver.
The full-duplex wireless communication scheme—or the in-band full-duplex as it has been described in some of the published literature—has been recently drawn much of attention by the academia and industry as a replacement of the currently deployed half-duplex wireless communication systems. The full-duplex systems are targeting a complete reuse of the utilized frequency band in order to exploit the limited spectrum resources more efficiently. The two full-duplex nodes, for example in a point to point scenario, will communicate over the same frequency band and without any time discontinuity. That is how the full-duplex wireless communication system is supposed to work, therefore, it will interfere itself by its own transmission signal which it is called in the literature the self-interference signal. Among wireless communication researchers, there is a broad consensus that the self-interference cancellation is the key enabling methodology for the full-duplex scheme. The realization of the self-interference cancellation has been visited in the literature in many techniques and transceiver architectures. The diversity of the proposed techniques and transceiver architecture alterations were nonetheless not able to entirely satisfy the greedy needs of the self-interference cancellation requirements.
In spite of the fact that couple of the experimental testbeds has demonstrated almost perfect self-interference cancellation levels [1,2], they were impractical for real-world implementation and serve only academic purposes to proof the concept. The lack of having a fully operational full-duplex transceiver architecture with an appropriate self-interference cancellation mechanism motivates this patent application. None of the accomplished work up so far has offered a practical concept of a full-duplex wireless transceiver. The invention that is going to be described throughout the rest of this document, crystalizes the architecture of a futuristic agile full-duplex wireless transceiver and the successive self-interference cancellation algorithm that should run over it. Several alternatives of the agile full-duplex transceiver are going to be covered within multiple embodiments which are enabled by the successive self-interference cancellation mechanism.
Many techniques have been discussed in the published literature concerning the full-duplex topic. Additionally, some transceiver solutions have been proposed in patent applications [3,4]. All these techniques have attempted to cancel the self-interference as an essential step towards enabling full-duplex systems. In spite of the fact that all these techniques share a common objective of canceling the self-interference, they have practically been realized in many different ways. A concatenation of several techniques to construct a whole robust mechanism of self-interference cancellation was however used in most of the published work.
The radio frequency (RF) domain self-interference cancellation is compulsory in any full-duplex system. Additionally, the digital cancellation was implemented in most of the self-interference cancellation mechanisms to suppress the self-interference furthermore. A tree chart categorizing the self-interference cancellation techniques that have been covered in the state-of-the-art is shown in FIG. 9. The cancellation techniques can be divided into two main categories: Cancellation at the RF domain and digital cancellation applied to the digitized samples. The RF cancellation techniques have been done either passively by attenuating the self-interference signal or actively by adding a self-interference cancellation signal to the RF interference signal (the signal-injection approaches as it called in the tree diagram in FIG. 9).
The RF attenuation cancellation techniques have been investigated in the literature [5,6,7]. These techniques rely basically on the direction of the antennas with combination of some other methods, such as antennas physical separation, dual polarization or RF absorbing materials. These techniques have experimentally demonstrated good passive cancellation results for the line-of-sight (LoS), whereas they were vulnerable against the rest of multipath wireless channel components. One main drawback of these canceling techniques, though, is that the uplink and downlink of the full-duplex node don't occupy the same spatial domain. Instead, they are directed towards two different locations. This makes such technique not applicable in point-to-point scenarios where both communication nodes are operating in full-duplex mode.
The signal-injection self-interference cancellation has diverse approaches based on how the self-interference signal is generated. For instance the work in [8,9] and [10] has proposed a self-interference cancellation technique based on a conditional placement of an antenna-set. This cancellation technique involves two transmitting antennas spaced apart from the receiving antenna by distances d and (d+λ/2) respectively. In that way the two transmitting antennas cause a null by superposition at the receiving antenna location. This mechanism suffers from many practical and performance limitations. One of these limitations is the placement calibration among antennas, which should be very accurate to ensure that the signal, which is impinging at the receiving antenna from the second transmitting antenna, is phase shifted by 180° degrees exactly. Even under the assumption that the calibration process is physically possible, this technique provides suppression of the self-interference signal at the center frequency. The suppression value is dramatically reduced when the frequency drifts away from the center frequency. As expected and proved by experiments, this cancellation technique works well only for narrowband systems.
Further work in [11] attempted to overcome the aforementioned drawback of the self-interference cancellation in [8] and additionally to reduce the number of the antennas that may be used. Authors in [11] have introduced an element to the full-duplex system design which is the RF Balun. By integrating the RF Balun into the full-duplex transceiver body, a negative version of the self-interference signal has been produced. With the aid of a noise canceling chip, the attenuation and delay that may be used were applied to the cancellation signal. The results of the RF Balun implementation are much better than the earlier work with antenna placement, particularly in broadening the self-interference cancellation bandwidth. Although, this approach still falls short of the self-interference cancellation requirements even with a consecutive stage of digital self-interference cancellation. Moreover, this approach has practical limitations such as the additional nonlinearities that the noise canceling chip introduces to cancellation signal and the RF Balun imperfections, such as leakage and frequency unflatness.
A totally different cancellation technique has been proposed by the scientists at Rice University [7,12,13]. This technique has been characterized in the literature as active cancellation technique—categorized here under the signal-injection with auxiliary transmission chain approach—due to its mechanism in which the full-duplex transceiver involves an additional transmission chain. The auxiliary transmission chain self-interference cancellation approach provides an appropriate room in the digital domain to implement and test several sophisticated digital signal processing algorithms in which the multipath self-interference wireless channel is considered in the waveform of the self-interference cancellation signal [14,15]. In spite of the flexibility which this cancellation technique has established by including the self-interference wireless channel in its multipath general model, this technique has some harmful consequences on the self-interference cancellation mechanism. Some of these consequences have been studied earlier in the literature. As a matter of fact, some of them have been characterized as the bottleneck in the active cancellation mechanism. The phase noise of the local oscillator is one of these effects which are limiting the performance of the auxiliary transmission chain cancellation approach [16,17], even though the same local oscillator is used for both transmitting chains. Another effect, which is usually neglected in the half-duplex systems, is the transmitter-generated noise [18]. Normally, in the conventional half-duplex system the receiver is located remotely at the other side of the communication. In contrast for the self-interference case in the full-duplex system, the receiver of the self-interference is located at the full-duplex node. Therefore the transmitter noise level will not lie below the noise floor of the receiver like in the case of half-duplex systems.
Recently, some extensible work from researchers at Stanford university has demonstrated promising results in the full-duplex communication systems [1,2,19]. These results have shown that in some scenarios the full-duplex systems have the potential to achieve a spectral efficiency as double as the half-duplex systems. This cancellation technique is based on printed circuit board (PCB) with multiple routes, having a different length in order to provide several delays. These multiple routes are supported with adjustable attenuators. The entire design is used to imitate the circulator leakage and the antenna mismatch reflection. This technique with a concatenating stage of digital self-interference cancellation is able to suppress the self-interference almost to the receiver noise floor. Many other considerations should be taken into account in the commodity wireless hardware such as having relatively nearby obstacles around the full-duplex node or having a compact transceiver design accommodating inside the full-duplex device. Additionally, the complicated structure of the transceiver as the system scaled up into MIMO configuration due to the RF circulator limitation, and the large number of PCB boards that may be used [2].
The digital self-interference cancellation may be used to suppress the residual self-interference further more. The diversification of the digital cancellation approach has come after the transmitter-generated noise limiting factor had been discovered. The solution with aids of additional receiving chain has been introduced to the literature in [18]. The auxiliary receiving chain has been employed to down-convert and digitize the self-interference signal, which is used to suppress the self-interference signal in the digital domain. Such technique outperforms the pure digital one, which relies on the digital baseband samples, by having a digital cancellation signal mixed with transmitter-generated noise.
In any two-way wireless transceiver, the transmission chain generates the RF signal to be transmitted wirelessly to the other node of the communication link. Meanwhile, the same node has to listen to the other side of the wireless communication link in order to receive the desired remote signal. The active transmission signal would interfere the reception of the remote signal and prevent the transceiver from receiving when it's actively transmitting. This problem has been solved so far by one of the duplexing schemes, which utilizes either two neighboring frequency bands or different time slots. These conventional schemes—frequency division duplexing (FDD) or time domain duplexing (TDD)—waste the limited frequency-time resources, therefore, the idea of having a system utilizes the time resources continuously and over the same frequency band is proposed recently [20]. This scheme of wireless duplexing is called the full-duplex or in-band full-duplex, not to be confused with the generic term of two-way type of communicating. In the in-band full-duplex, the communicating nodes transmit and receive over the same frequency band and they are simultaneously active all the time, of course when it's needed. Such duplexing scheme is supposed to offer the best utilization manner of the frequency time resources so far however, it's not easy to be practically realized.
In practice, the transmission chain in a full-duplex transceiver generates the RF transmission signal which is supposed to be received by the remote communication node. Unlike in the FDD transceivers where this transmission signal would be suppressed by means of RF duplexing filters, this signal is entirely received by the transceiver itself and interferes the reception in the local transceiver. This interference signal is called the self-interference signal, which is the main obstacle that have to be tackled in order to practically realize the full-duplex scheme. The desired reception signal is vastly attenuated due to the long travelling distance in the wireless medium, whereas the self-interference is locally generated and therefore is really less attenuated than the remote signal. Hence, the self-interference signal prevents the transceiver from receiving the remote desired signal due to its overwhelming magnitude.
Accordingly, the self-interference signal has to be sufficiently suppressed in order to be able to receive the remote desired signal. The amount of suppression that may be used is high, which might be not possible to be suppressed by a simple cancellation technique, especially when a compact transceiver specification has to be maintained. Another technical problem that the currently-deployed FDD wireless transceivers suffers from, which is the out-of-band emissions. These emissions are unwanted, and they spill from the transmission frequency band over the receiving one—up-link to down-link or the other way around depending on the node type, user terminal or base station. This stresses the design constrains of the diplexer filters in order to meet a highly attenuation factors for the out-of-band emissions. Additionally, it imposes the need of placing a duplexing gap between the up-link and the down-link frequency bands.
Thus, there is a need for an enhancement of wireless communications.